Yes. As I recall it, the fire was caused by downed lines that were energized during a dry spell, and the reason the lines were downed was due to negligence around maintaining the C-Hooks holding their high voltage transmission lines. PG&E knew that they needed to be replaced every so often, had a policy that dictated when they needed to be replaced, and then ignored that policy which ultimately allowed a C-Hook to fail and the energized line to start the fire. In fact, PG&E had commissioned a study as far back as 1987 to look into this issue and confirmed that these hooks had a limited lifespan.
They had clear knowledge of the issue. They had a responsibility to maintain the system to prevent the issue. They set policies around how that maintenance should be conducted. Then they willfully ignored their own policies, which lead to the issue they were responsible to prevent. That's textbook negligence.
So, yes, PG&E /did/ cause the fire. They were negligent in doing so. They are liable for the damages.